This invention relates to controllers similar to the type known in the metal stamping industry as malfunction detectors which are used to monitor and stop various automatic stamping processes upon reception of a machine-element or work-piece malfunction signal. In particular, this invention relates to controllers suitable for use in conjunction with continuous cycle punch presses or the like for monitoring the process to prevent or decrease machine, tooling, and work-piece damage, and also to initiate or control various steps or functions of the process.
Continuous process metal stamping machines (e.g., punch presses equipped with progressive or transfer dies which form and punch out integral parts from a continuous strip of material) are subject to inordinate shutdown and expensive repair or replacement of tooling when an anomaly occurs in the processing cycle such as: mis-feed (mis-registration of the work-piece or strip), mis-position of machine element (broken spring or actuator), part not ejected, etc., and the machine is not stopped before the next strike (die closure). Since these machines are usually run at speeds ranging from 50 strokes per minute to 150 strokes per minute, a human operator is not physically able to react to a malfunction and stop the machine before damage occurs, especially as the most potentially damaging problems arise from process malfunctions occurring inside the dies and usually not visible to an operator.
The evolution and use of malfunction detectors as bolt-on equipment peripheral to punch presses and the like has advanced through manufacturers' need to reduce occurrence and costs of damaged tooling, and the reluctance of machine manufacturers to enter the field of process control due to installation and maintenance demands, and the wide variations of processes which can be served by such equipment.
Early malfunction detectors were crude and consisted of a pair or more contacts or switches actuated by the work-piece and a machine-element connected in a series/parallel arrangement with the punch press RUN relay control power. This arrangement often exposed the set-up man or machine operator to electric shock hazard.
Later malfunction detectors provided individual channels, each comprised of sensor, go/no-go circuit, lamp indication and relay control of run control power. Timing and acceptance of sensed signals were accomplished by charge/discharge of resistance-capacitance (RC) networks which are influenced by component age, temperature, and line voltage variations. There was no provision for sensing failures occurring in the internal circuits of the malfunction detector.
More recently, some malfunction detectors were manufactured with solid-state components and have rudimentary circuit failure sensing means. In these embodiments, however, it is still possible for the monitored machine to continue running for one or more strokes after a circuit failure has occurred, and, in which case, the punch press stop-point is generally indeterminate due to the continued practice of using RC timing circuits, allowing tooling damage to occur in some instances.
Present tooling in the metal stamping industry has become more complicated and thereby more costly, and the need for increased production has raised punch press speeds with a consequent demand for better protection of tooling.
A continuous-feed punch press is comprised, basically, of a motor and controls, a crank shaft with an attached brake and clutch, a moveable ram connected to the crank shaft, a fixed bed, and a rigid frame to hold these parts in an aligned, working arrangement. During normal operation, the main shaft is continuously revolved in one direction and the crank-connected ram moves up and down repetitively in response. A die-set containing various punching and forming tools is fixed to ram and bed. A means of feeding strip material (often steel) into the die-set is ordinarily a pair of smooth metal rollers which grip the strip and, powered from the main shaft by a crank and rack and pinion gear, advance the strip on alternate half-revolutions of the main shaft through a unidirectional clutch. The feed mechanism is usually adjusted so the feed occurs half during the up-stroke after the die opens and half during the down-stroke before the die closes on the strip. A cam operated roller release mechanism causes the rollers to grip the strip during the feed operation and to release the strip during the punching part of the machine cycle.
In operation, after the strip has been advanced, and before the feed rollers release, pilot members of the die enter pilot holes in the strip which holes were punched in a previous stroke. The feed rollers then release and as the die closes, the pilots register the strip in the die accurately. Punching and forming members do their work as the die continues to close.
On the up-stroke, the die begins to open, the feed rollers grip the strip and after the pilots emerge from the strip, the rollers begin to advance the strip.
In progressive dies, the number of stations (die segments) the strip progresses or advances through increases in direct proportion to the complexity of the part being fabricated. In this type of stamping operation, the feeding of strip material is a very important function and parameter, and should be monitored because if for some cause, such as roller slippage, or blocking in the die of the progression of the strip by a piece of scrap material, etc., the strip fails to advance far enough for the pilots to enter the pilot holes, the pilots (of blunt, bullet-nosed shape) will deform the strip and may split the die section under them at the point of impact as the die closes.
For the above circumstance, feed completion is monitored by a mis-feed malfunction detector channel and if the strip does not advance the correct amount, the protective circuits will stop the machine before the pilots enter the strip.
In punch presses as described above, in which feeding occurs over an arc of 180.degree. of one revolution, it is advantageous to begin the feed as early on the up-stroke as possible so that feeding is completed as early on the down-stroke as possible in order to give enough time for the ram to be stopped by release of the clutch and actuation of the brake before the pilots enter the strip in the case of a protective stop on mis-feed.
A mis-feed channel could be termed "end of feed detector," because the signal of importance would occur at or very near the time of the end or completion of the feed advancement. Such a channel would have a single sensor which could be an electrically insulated metal probe with an exposed metal tip positioned in or near the die so as to make contact with the metal strip only when the strip is advanced the full feed increment. The probe would be connected by wire to one terminal of a low voltage relay coil, the other terminal of which connected to one terminal of a low voltage source. The other terminal of the low voltage source would be connected to common ground. The frame of the machine is also connected to common ground. Thus, when the strip contacts the probe, the circuit for the coil is completed through metal on metal contacts: probe to strip, strip to die, die to bed, bed to frame. So energized, a set of N.O. (normally open) contacts on the relay close, which contacts are wired in parallel with a set of contacts (N.O.) of a cam operated switch which are being held closed by a cam positioned on or geared to the main shaft of the machine. The cam is so shaped as to hold the switch contacts closed until just after the end of the feed occurs, at which time the cam releases the switch for some period corresponding to a definite shaft angle. This shaft angle is determined in part by the point in the cycle at which the projecting strip (now a formed part) is cut off as a finished piece and thus breaks contact with the probe. The parallel set of contacts described are wired in series with the power to the "run" control circuit of the punch press. If both contacts open at the same time, the machine stops. Therefore, if on every machine cycle, the feed probe makes continuous contact with the strip while the cam operated switch is open, the machine will continue to operate. In this case of a mis-feed, when the cam operated switch opens, the relay not having been energized by probe to strip contact, has open contacts also and the machine is stopped by loss of power to the run control circuit. Optimumly, the probe and cam and the feed cycle have all been adjusted for best performance and the punch press stops before any damage is done.
In the above example, the machine continues to run until a missing end of feed signal, hence: a miss.
One draw-back of the above circuitry (equivalent to the early forms of malfunction detectors) is the false signal. A false signal would be generated in the above example by a piece of scrap or other metal continuously maintaining a connection between the probe and common ground (die, bed, or frame of machine). In such a case, when the cam operated switch opened to check the end of feed, the scrap metal continuously provides an "OK to run" (RUN) signal regardless of the feed condition. If under these circumstances, a mis-feed occurs, the punch press would continue to run and tooling damage would occur. There is no inherent protection in the above described circuitry to prevent such an event from bypassing the purpose of the mis-feed detector channel. An operator, due to some misunderstanding of the purpose of such a detector, and not having success in getting the machine to run continuously with it due to a possible failure in another part of the circuit, e.g., faulty cam switch, cam loose or misadjusted, loose or broken wire, etc., may choose to purposely bypass the circuit by simply connecting the probe or its connecting wire to common ground.
Another draw back of the above circuit is the fault condition. A fault signal would occur when the cam operated switch failed in the closed position. In such a case, the switch contacts maintain Run control power continuously and, although the probe and its associated relay work consistently, the cam operated switch contacts never open to allow the end of feed signal presence to be checked. Therefore, on a mis-feed, a fault signal will allow the punch press to continue to run and again tooling damage will occur.